Applied Catalysis A: General 265 (2004) 229–235 Low-temperature adsorption of oxygen on calcined chromia: IR spectroscopic and sorptiometric evidence for oxygen-assisted topochemical reduction of surface chromate species Radamis B. Fahim a,1 , Mohamed I. Zaki a,∗ , Nasr E. Fouad a , Mohamed Abdel-Khalik b , Norman Sheppard c a Chemistry Department, Faculty of Science, Minia University, El-Minia 61519, Egypt b Chemistry Department, Faculty of Science, Ain Shams University, Egypt c School of Chemical Sciences, University of East Anglia, Norwich NR4 7TJ, England, UK Received in revised form 30 December 2003; accepted 16 January 2004 Available online 20 March 2004 Abstract The evolution with calcination of chromia samples of catalytic interest, prepared from gels with or without added ammonium nitrate, has been studied by thermogravimetry (TG) and differential thermal analysis (DTA). The low-temperature (77K) adsorption of oxygen on alumina- and silica-supported samples of the former type has revealed the presence of monolayer and multilayer types of irreversibly adsorbed species. Low-temperature in situ infrared (IR) spectra show that the adsorption of oxygen is associated with the removal of a multiplicity of absorptions between 1100 and 730 cm -1 . Probable types of surface species removed are Cr=O groups of Cr(VI)–O surface species, dichromates and chromates together with possibly peroxide; (O 2 ) 2- , the latter being associated with prominent bands near 800 cm -1 . Room-temperature ex situ infrared spectra on an alumina-supported sample shows at lower frequencies the presence of the growth of -Cr 2 O 3 surface species after oxygen adsorption. © 2004 Elsevier B.V. All rights reserved. Keywords: Low-temperature oxygen adsorption; Calcined chromia; Surface chromate reduction; In situ infrared spectroscopy; Sorptiometry 1. Introduction Calcination (heating in air or in pure oxygen) of unsup- ported, or silica- or alumina-supported, chromia (Cr 2 O 3 ) at 873 K generates surface Cr(VI)–O species [1–4]. These species are monomeric and/or polymeric chromates [(CrO 4 ) 2- and/or (Cr 2+x O 7+3x ) 2- ] [5,6] anchored onto surfaces of the crystalline or noncrystalline chromia phase or onto surfaces of the support material [7]. When allowed to interact electronically with nearby Cr(III)–O species [6,7], the chromates are stabilised with respect to thermal decomposition, hydrolysis or chemical reduction [3,6,8]. The availability of electronically-coupled Cr(III)–Cr(VI) species nevertheless provides the electron-mobile environ- ment required for surface redox reactions [1,2,9]. Whereas, localised adsorption occurs on coordinatively-unsaturated ∗ Corresponding author. Fax: +20-86360833. E-mail address: mizaki@link.net (M.I. Zaki). 1 Deceased. Cr(III) sites, the electron availability occurs through the interaction with nearby Cr(VI) ions [7,10,11]. Accordingly, calcined chromia catalysts have shown potential in a range of important redox reactions, such as CO oxidation [11], dehydrogenation of alcohols [1], and H 2 O 2 decomposition [9]. The elimination of unstable chromates by a brief CO-reduction at 623 K [12,13], or by hydrolysis at room temperature [14,15], does not significantly alter the sur- face redox activity [11]. It is worth noting that the high volatility of Cr(VI)–O compounds has led environmental- ists to classify these compounds as detrimental within the atmosphere [16]. For this reason industrial applications of calcined chromia catalysts have been largely hampered [9], despite their success in numerous laboratory applications [1,2,17,18]. However, chromia catalysts synthesised by calcination and a subsequent elimination of unstable chro- mates, as described above, would still possess sufficient catalytic potential in redox processes while posing no acute threat to the environment [8]. 0926-860X/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2004.01.022